Dual purpose air diffuser for jet engines



Dec. 26, 1961 E. Q. SMITH, JR., ET AL 3,014,426

DUAL PURPOSE AIR DIFFUSER FOR JET ENGINES 2 Sheets-Sheet 1 Filed June 27, 1957 l l u (\l I INVENTORS E. QUIMBY SMITH JR.

WILLY A. FIEDLER BY {L 3LQ7'TORN Y5 Dec. 26, 1961 E. Q. SMITH, JR., ET AL 3,014,426

DUAL PURPOSE AIR DIFFUSER FOR JET ENGINES 2 Sheets-Sheet 2 Filed June 2'7, 1957 INVENTORS E. QUIMBY SMITH JR. WILLY A. FIEDLER TOR/VEYS 3,014,426 Patented Dec. 26, 1961 The invention described herein maybe manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention in a preferred embodiment relates to supersonic missiles propelled by ram-jet or turbojet engines, the missiles also incorporating some form of targetseeking or homing apparatus'responsive to the reception of radiant energy emanating from the target itself.

A ram-jet or turbojet engine of standard design includes a compression intake .made up of a central needle portion, or spike, surrounded byan essentially tubular casing or jacket which is positioned so as to be spaced from the spike. This forms an annular air duct defined by the outer surface of the needle member and the inner surface of the tubular casing or jacket. While the spike may have various rotary-symmetrical shapes, it is most frequently of conical or frusto-conical configuration.

In supersonic ram-jet. performance, the effect of compression due to shock waves is an important consideration.

" As the speed of an engine in the supersonic range is increased, the angles of the resulting shock waves, with relation to the longitudinal axis, are decreased. To obtain themost efficient ram compression ratio, the shock wave convergence must be maintained within the adjacent forward lip of the ram inlet. The curved surfaceo'f the forward needle portion (or spike) should therefore be such as to bring about ashock wave convergence in the vicinity of the air inlet at a predetermined missile speed. Furthermore, the geometry of the spike (fora pressure recovery ratio approaching one) should be such that a large number of weak shocks are developed rather than a few strong ones. The above dictates a spike configuration incorporating a minimum discontinuity in difiuser angle.

which is designed in hemispherical form for optimum collection efiiciency, even though such a configuration results in considerable aerodynamic drag. However, re.- placement of the hemispherical irdome by aspike or ogive, while reducing aerodynamic losses, may cause impairment of target'resolution and seeker range by adversely afiecting the signal-to-no-ise ratio of the optic collector.

of collecting the electro-magnetic energy which passes through a translucent irdome, the energy is first collected and then focussed through the translucent member. This approach to the problem permits a single unit to perform the dual function of air diffuser and radiant energy collector.

In order to accomplish the above, the needle or spike portion of the nozzle assembly is so configured as to define one boundary of an annular air duct. The outer tubular jacket, or cowl, constitutes the other air duct surface. In addition, these two members not only make up the missile diffuser, but also act as complementary reflectors for radiant energy arriving from the projectile target. This incident energy, in one embodiment, is reflected first from the spike surface to the cowl, and then again reflected from the latter to a focal point lying along the missile axis. Since such focal point is located within the spike, that portion of the spike body within the region traversed by the twice-reflected energy is composed of translucent material, permitting relatively unimpeded passage therethrough of radiant energy to the point of collection. With such a design, very little of 'the available energy is lost, and the assembly consequently displays an unusually high optical signal-to-noise ratio, as well as functioning as an elfective air diffuser for the missile propulsion system.

One object of the present invention, therefore, is to provide a combined air diffuser and radiant energy collector for projectiles.

Another object of the invention is to provide a combined air dilfuser'and radiant energy collector unit for missile engines of the ram-jet type, this unit being so designed that radiant energy is sequentially reflected from a plurality of diffuser surfaces and then brought to an approximate focal point on or near the missile axis;

A further, object of the invention is to modify the design of a jet engine air diffuser assembly so as to permit its use as a collector of radiant energy without appreciably reducing its aerodynamic efficiency.

Other objects and many of the attendantadvantages of this invention'will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

When an infrared homing missile of-the above class is powered by a ram-jet or turbojet engine, the hemispherical irdome must be integrated into the ram-jet air intake. This causes a marked reduction in diffuser efficiency. On the other hand, attempted modification of the hemispherical irdome to maintain diffuser efiiciency has hitherto precluded an optimum collection of infrared energy.

The principal objective of the present invention is to provide an efiicient diffuser design for ram-jet or turbojet engines, which design is also suitable for eflicient radiant energy collection. Basically, this objective is achieved by reversing the normal collector process-that is, instead the longitudinal axis13 of the missile.

FIG. 1 is a longitudinal section of a jet-propelled projectile incorporating a radiant-energy-collection system of a design known at the time the present invention, was conceived;

FIGS. 2, 3, 4, and 5 are schematic diagrams illustrating the optical principles on which the. present concept is founded;

. FIG. 6 is a partly schematic illustration of a combined airdifiuser and optic collector basedon the principles developed in FIGS. 2 through 5;

FIG. 7 is a modification of the combined air diffuser and optic collector, of. FIG. 6; and v J FIG.- 8 is a longitudinal section of a jet-propelled projectile incorporating another modified type of combined air diffuser'and optic collector in accordance with the present invention.

Referring now to the drawings, there is shown in FIG.

1 a ram-jet missile incorporating a radiant energy collector of a type known prior to conception of the invention disclosed herein. This missile includes a cylindrically-shaped shell 1t and a forwardly-located inner member 12 defining a surface of revolution relativeto These two elements 1t) and 12 act in a known manner as a diffuser tion of the missile form no part of the present invention and further discussion thereof is believed to be unnecessary.

It has previously been established by workers in the projectile field that an optimum aerodynamic design for the forward portion of the inner diffuser member 12 incorporates a pointed tip which flares rearwardly with minimum surface discontinuity to a point within the lips 16 of the outer shell 10. This reduces the amplitude of the developed shock waves and increases the pressure recovery ratio of the diffuser section. Such a design is therefore widely employed for ram-jet assemblies of the non-homing type.

However, when the projectile is so constructed as to seek out a source of radiant energy, the aerodynamic considerations set forth above must be modified to take into account the particular requirements of a homing missile. These requirements include the ability of the missile to intercept incident energy from the target or source, and to convert amplitude variations in this received energy into control information. Generally speaking, the greater the peak-to-peak amplitude of the usable control information developed, the more accurately the missile may be maintained on target. 7

The above considerations therefore necessitate a radiant energy collector which is of such dimensions, in a plane normal to the longitudinal axis of the missile, that a sizeable bundle of energy radiations are intercepted and directed to some form of transducer for conversion into control information. In general, the greater the cross-sectional area of this bundle of rays, the higher will be the signal-to-noise ratio of the developed error voltage.

Inasmuch as the best, if not the only, portion of the missile which can adequately serve as an energy collector is the forward nose section, it has been customary to construct a ram-jet missile of the homing type with an irdome acting as one of the elements of the diffuser assembly. As shown in FIG. 1, the forward portion of the inner member 12 is composed of some suitable translucent material 13 which permits passage therethrough of energy rays arriving along the paths 20. The rays, after passing through the translucent section 18, are sequentially directed by a plurality of reflectors 22 to a focal point 24 lying on the longitudinal axis 13 of the projectile. The arrangement of the reflectors 22 is that of a conventional Cassegrain optical system. All or a part of the reflector assembly 22 may be mounted for limited pivotal movement about a point on the axis 13 to permit the missile to search out or track a target when the reflector assembly is actuated by a suitable source of power (not shown). In any event, the function of the optical system and its associated circuitry-is to develop an error signal which is effective to maintain a parallel relationship of the missile axis 13 with the light rays, which is equivalent to maintaining the missile in a path aligned with the desired target.

While an arrangement such as set forth abo e has generally satisfactory energy-collection characteristics, it is relatively inefl'icient aerodynamically. This results from the necessity of constructing the irdome of essentially hemispherical configuration primarily in order to provide sufiicient space to contain the optical system 22. It has been found that large shock waves are developed due to the marked discontinuity between the surface of the irdome and that of a needle 25. It is not feasible to minimize this discontinuity by appreciably increasing the thickness of the translucent window 1% in this region, since such an expedient results in excessive scattering and/or refraction of the radiant energy, and an appreciable drop in the number of rays reaching the focal point 24.

It is the principal objective of the present invention to eliminate the above drawbacks, and provide a combined jet engine air di fusing unit and radiant energy collector which performs both functions with high efficiency. As will be seen from the description which follows, the arrangement of the present invention reduces the number of components required by permitting the complex Cassegrain reflector system of previous designs to be dispensed with.

The principle on which the present invention is based is more or less schematically illustrated in FIGS. 2 through 5. It will be recalled that in the known design described above, the radiant energy information is obtained by providing a translucent projectile nosesection through which the rays pass with negligible change in either direction or intensity. The rays are then collected by an optical system and focused to a point which lies on the missile axis. The system of the present invention in effect reverses this procedure by collecting and focusing the radiant energy before it passes through the translucent section of the air diffuser unit. In other words, such collection and focusing occur extreiorly of the diffuser member within which the transducer is located, this diffuser member also preferably containing other electronic components of the missile midance system.

It has been found that such a method of radiant energy collection permits the missile nose section to be designed for optimum aerodynamic efficiency. From FIG. 2 it will be seen that, if the parabolic surface ABC is symmetrical about the axis XX, then all arriving energy rays such as D and E which are parallel to the axis X-X will be focused to a common point P.

No. assume that a further line YY be drawn through point C and parallel to the axis XX, and that a still further line be drawn normal to X-X and from a point G thereon to intersect the parabolic surface ABC at H.

When the area CGH of FIG. 2 is rotated about YY as an axis, the solid of revolution CHGH of FIG. 3 results. The outer surface of the inverted paraboloid of this latter figure is the optical counterpart of the parabolic surface ABC of FIG. 2, in the sense that the parallel energy rays D and B will be brought to a focus at F, but now the reflected rays will not generate a point focus, as in FIG. 2, but will instead form a focal ring.

It is intended that the solid of revolution CHGH of FIG. 3 be employed as that part of the air diffuser assembly represented by the hemispherical irdome of FIG. 1. From the standpoint of its function as an air compressor, it has an ideal geometry, inasmuch as it is formed by rotation of the parabolic section CH, which for this purpose may be considered as defining a continuously varying angle which produces minimum shock effects in the air entering the intake area 13a. From an optical standpoint, it brings about a focus on a ring F of those energy rays arriving parallel to axis YY of FIG. 3. Althous it is conceivable that the energy re flected to this region could be picked up along the focal ring, the fact that such reflected energy is spread out rather than concentrated renders such collection impractical, and in any event the apparatus for accomplishing such a result would be excessively complex for projectiles already encumbered with the involved circuitry necessary for accurate electronic guidance.

Consequently, the energy reflected from the surface of revolution CH in FIG. 3 is redirected to a focal point lying on the longitudinal axis YY of the missile. The means for bringing about such a result is shown in FIG. 4, and includes a second surface of revolution such as might be provided'in modified form by the hollow cylindrical body shell in of FIG. 1. The essential characteristic of this second reflecting surface is that the focal ring must be imaged to a focal point in the manner brought out by FIG. 4that is, the contour and spacing of the second reflecting surface relative to the first reflecting surface must be such that the virtual focal ring F is imaged at a real focal point P lying on the missile axis Y -Y Such a result may, for example, be achieved by em- -.ploying as a secondary reflector the right section surface seem to indicate that greater optical collection efliciency results from use of the parabolic-elliptic configuration, there are a number of aerodynamic benefits to bederived from the parabolic-conic combination due to less severe gradients in the air flow directional vectors, The actual shape chosen, therefore, may represent a working compromise between the two designs.

In FIG. 6 is illustrated a modification of a portion of the projectile of FIG. 1 to incorporate a preferred form of the present invention. It is based on the parabolicelliptic showing of FIG. 5 in that the surface portion CH of the inner body member 12 is that of an inverted paraboloid of revolution. The inner surface of the lip portion 16 0f the outer shell 10, on the other hand, is that of an ellipsoidal segment. All-energy rays 20 parallel to the missile axis will be sequentially reflected from the inverted parabolic outer surface of the pointed nose section 11a and then from the ellipsoidal inner surface of the cowl section 16 to converge at the focal point 24.

For a minimum energy loss, the portions of the air diffuser assembly from which the rays are sequentially reflected should be highly polished and contain substantially no surface irregularities. To permit passage to the focal point 24 of the energy rays which have been reflected from the cowl surface 16, a portion (18a) of the inner body member 12 which lies adjacent the inverted parabolic section 12a is formed of translucent material. It may, for example, be composed of a substance similar to that of which the entire hemispherical irdome 18 of-FIG. l is formed. It should also be of minimum thickness consistent with air pressure requirements in order to preclude excessive diifusion and/ or refraction of the radiant energy between the entrance and emergent surfaces. 7 j

As illustrated in FIG. 1, an electro-optic-al transducing 7 device 26 is located within the inner body member 12 so as to intercept the energy rays converging at the focal point 24 and to translate such rays into electrical variations. The latter are applied to any suitable electronic guidance system 28 which is associated with the missile flight controls and hence is adapted to utilize variations in magnitude of the radiant energy intercepted by transducer 26 to effect a correctional change in direction of the missile whenever the latter deviates. from an on target flight path. 1

' .Although the above discussion has presupposed that the radiant energy rays are arriving at the collector along paths-which are substantially parallel to the missile axis,

it is obvious that for a homing" missile the principal intere st lies in the ability of the seeker apparatus to develop usable error voltages from incident energy which is off-axis. It might be assumed that under such conditions the position of the focal point 24 would shift from the location described and cause a marked'decrease in that the, above assumption is unfounded. Measurements taken as the respective angles of incidence of the energy rays vary widely show that an appreciable fall-off in rela- FIG. 1, the parabolic nose section 12a of FIG. 6 may be loosely instead of rigidly supported so that it has limited universal movement over an angle of approximately 3 degrees or less with respect to the missile axis 13. This arrangement is illustrated in FIG. v7. The rear portion of this nose section may be provided with several small ears or tabs 29 which project into the air stream flowing within'the duct 13a. v The effect of this arrangement is to cause a relatively fast oscillation or vibration of the nose section (100 cps. is an example) about the pivot point 29a, changes the angular relationship of the parabolic surface of the nose section 12a with respect to the energy rays arriving along 'the paths 20. Information as to the instantaneous angular position of the nose section may be obtained through conventional capacitative or electromagnetic coupling, and transmitted to a standard phase compacitor (not shown). nose section of the diffuser unit relative to the cowl causes the missile to track the target much in the manner of a search radar, and markedly increases the effectivenes of the energy collector assembly.

When the nose section 12a is mounted for oscillation in the manner illustrated by FIG. 7, the translucent wall portion 18a employed in the assembly of FIG. 6 may be omit-ted so that the nose element is free to move aboutv the pivot point 2% within the limits mentioned above. Contrary to expectations, it has been found that substitution of an air gap for the translucent material of FIG. 6 has no appreciable effect on theair flow distribution over the surface of the inner member 12, since the air gap acts as a free jet boundary at high projectile speeds. For the same reason, it may in some cases be desirable to employ such an air gap even though the forward nose section 12a is fixed in position as in the embodiment of FIG. 6. i 1

It should be emphasized that the present invention incorporates the reflection and focusing of radiant energy by one or more sections ofa missile air diifuser unit prior to entrance of such reflected and focused rays into the body portion of the missile in which the energy transducer is located. Although certain geometrical configurations for these reflecting and focusing surfaces have been set forth as preferred examples, they are merely representative of many such sets or combinations of said configurations which will bring the radiant energy to such a common point. For example, the spike portion of the air diffuser has been described as parabolic and the cowl portion as conical. However, if the spike were required or desired to be conical, then the cowl contour could be a segment of a parabola the focal point of which is located on the missile axis. Many other contour combinations are also possible, and their selection and use will depend upon missile design factors and operating conditions.

While theinvention has been illustrated and described as applied to apilotless missile adapted to carry an explosive charge, it likewise has utility in connection with commercal jet propelled arcraft for the purpose of avoiding mid-air collisions with moving or stationary objects lying in the aircraft flight path. To this end the missile guidance system previously described could be replaced by a radar transmitter-receiver located wholly or partly within the inner body member 12 of the jet engine air diffuser, with the transducer or feed-horn positioned at or near the focal point 24. When the radar transmitter is pulsed in conventional fashion, lack of any returned signal would indicate a clear flight path. Reception of 'marked simplification of ordinary radar installations, both tive intensity (for the parabolic-ellipsoidalcombination) 'does not occur until the angle made by the energy rays with the missile axis exceeds approximately 26 degrees. 1 Furthermore, as an analogyto thepivotal mounting for the reflector system 22 of the known arrangement of energy, on the other hand, would warn the pilot of the possibility of an impending collision with an object in the aircrafts proximity. Such an arrangement would be a as to the number of circuit components employed and as to space and weight considerations.

In FIGJS of the drawings is illustrated another embodiment of the present invention in 'which radiant energy Such a pivoting of the v rays are reflected but once from a missile surface area prior to arriving at a common focal point. In this construction, energy rays having the parallel paths 2% first impinge the inner surface of the forward cowl portion 16 of the shell member 1ft. This inner surface is of generally parabolic configuration from the forward edge approximately to the region 3th of maximum throat constriction. Hence, energy rays following paths parallel to the axis of the missile will be focused at point 2 4 regardless of where they may impinge the parabolic inner surface of the cowl 16.

he inner member 12 in FIG. 8 is of relatively small diameter compared to the corresponding member of FIG. 6, and has a forward portion 32 of radiant-energy-permeable material which may be similar to that of the wall portions Eda. One advantage inherent in the arrangement of FIG. 8 is that elimination of one reflecting surface correspondingly eliminates whatever diffusion and-scattering of energy rays would be caused thereby, and hence raises the percentage of recoverable target information. Incidentally, use of the term radiant energy in the above description is intended to embrace all forms of electromagnetic energy of any wavelength.

Obviously many modifications and variations of the present invention are possble in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim:

1. In a target-seeking missile powered by a ram-jet engine, a combined radiant energy collector and air diffuser for said engine, said combined energy collector and air diffuser including an outer essentially tubular cowling and an elongated inner body member both symmetrically arranged with respect to the longitudinal axis of said missile, said cowling and inner body member defining therebetween an annular air intake port for said jet engine, optically complementary means, including the forwardly-located surface portion of said body member and a portion of the inner surface of said cowling, for sequentially and in that order reflecting to a common focal point lying within said body member radiant energy rays arriving at the forwardly-located surface portion of said body member along paths directionally related to the longitudinal axis of said missile, and an annular radiant-energypermeable wall section formed in said inner body member in the region impinged by the rays reflected from said cowling to pass such rays to said common focal point, means located at such focal point for collecting said energy rays, and means for applying the energy derived therefrom to control the trajectory of said missile.

2. A target-seeking missile according to claim 1, in which the forward portion of the surface of said inner body member defines a segment of an inverted paraboloid.

3. A target-seeking missile according to claim 1, in which the forward portion of the surface of said inner body member defines a segment of an inverted paraboloid, and in which the forward portion of said cowling has an inner surface of substantially elliptical configuration.

4. A target-seeking missile according to claim 1, in which the forward portion of the surface of said inner body member defines a segment of an inverted paraboloid, and in which the forward portion of said cowling has an inner surface of substantially conical configuration.

5. An air diffuser unit for a missile having a ram-jet engine, said diffuser including an inner body member and an outer cowl member so spaced as to establish therebetween an annular jet engine air intake port, said inner body member and said outer cowl member each defining a surface of revolution with respect to the longitudinal axis of said engine, the forward portion of the surface of said inner body member being configured to reflect radiant energy rays, reaching the surface of such body member along paths directionally related to the longitudinal axis of said engine, to the inner surface of said cowl memher, the inner surface of said cowl being configured to be optically complementary to the configuration of the sur-- face of the forward portion of said inner body member so as to again reflect the radiant energy rays, reflected to it from the surface of said inner body member, to a com-- mon collection region lying within said body member, and transducer means located at such common collect-ion: region for developing from the energy rays so collectedan electrical variation suitable for controlling the tra-- jectory of said missile.

6. A combined air diffuser and radiant energy collector unit for a missile having an engine of the ram-jet type comprising a generally elongated inner member defining; a body of revolution relative to the longitudinal axis of said engine, an outer cowl member of generally tubular configuration, symmetrically disposed with respect to said inner member and spaced therefrom so as to define an annular jet engine air intake port therebetween, the surface configuration of the forward portion of said inner member defining in cross-section a substantially continuously varying angle relative to the longitudinal axis of said engine, whereby shock waves developed at said diffuser during progress of said engine through the atmosphere are minimized, the forward surface portion of said inner member acting to reflect radiant energy rays which reach this forward portion of said inner member along paths which are directionally related to the longitudinal axis of said engine to the forward portion of the inner surface of said cowl, the inner surface of said cowl being opticmly complementary to the forward surface portion of said inner member so that the former will act to again reflect and focus radiant energy rays reflected thereto from said inner membe to a common region lying within said inner member, means located at such common region to collect the energy rays focused thereto, and means for utilizing the energy rays so collected to determine the trajectory of said missile.

7. A combined air diffuser and radiant energy collector for a missile having an engine of the ram-jet type comprising a generally elongated inner member defining a body of revolution relative to the longitudinal axis of said engine, an outer cowl member of generally tubular configuration, symmetrically disposed with respect to the said inner member and spaced therefrom so as to define an an nular jet engine air intake therebetween, the surface configuration of the forward portion of said inner member defining in cross section a substantially continuously varying angle relative to the longitudinal axis of said engine, whereby shock waves developed at said diffuser during progress of said engine through the atmosphere are minimized, the forward surface portion of said inner member and the forward portion of the inner surface of said cowl each being configured to be optically complementary whereby such surfaces act to sequentially reflect to a point within said inner member radiant energy rays reaching the forward portion of said inner member along paths which are directionally related to the longitudinal axis of said engine, the forward portion of said inner member being fixed in position relative to said axis, means located at said point for collecting the energy rays so sequentially reflected, and means for applying the collected energy to determine the trajectory of said missile.

8. A combined air diffuser and radiant energy collector for engines of the ram-jet type comprising a generally elongated inner member defining a body of revolution relative to the longitudinal axis of said engine, a transducer located within said inner member, an outer cowl I -member of generally tubular configuration, symmetrically disposed with respect to the said inner member and spaced therefrom so as to define an annular jet engine air intake port therebetween, the surface configuration of the forward portion of said inner member defining in cross section a substantially continuously varying angle relative-to the longitudinal axis of said engine, whereby shock waves developed at said diffuser during progress of said engine through the atmosphere are minimized, the forward surface portion of said inner member and the forward portion of the inner surf-ace of said cowl acting to sequentially reflect to said transducer radiant energy rays reaching the forward portion of said inner member along paths which are directionally related to the longitudinal axis of said engine, and means for causing limited relative angular movement between said transducer and the forward portion of said inner member.

9. The combination of claim 8, in which the forward portion of said inner member is provided with a plurality of tabular extensions projecting into the said air intake port, said'tabular extensions reacting with the airfiowing through said intake port to cause an oscillatory movement of said inner member.

10. A combined air difiuser and radiant energy collector for engines of the ram-jet type, comprising a generally elongated inner member defining a body of revolution relative to the longitudinal axis of said engine, a generally tubular outer cowl member formed with a progressively decreasing diameter to provide a restricted passage, said area of decreasing diameter forming an air intake port forwardly of said elongated member, said outer cowl member being symmetrically disposed with respect to said inner member, the inner surface of said outer cowl member forwardly of the restricted passage reflecting radiant energy rays, initially impacting the inner surface of said outer cowl member after arriving at said inner surface along paths substantially parallel to the longitudinal axis of said engine, to a common focal point located within said inner member, said inner member being formed at least in part of radiant-energy-penne- 10 able material to pass therethrough the rays reflected thereto from the inner surface of said cowl.

11. A combined air diffuser and radiant energy collector for engines of the ram-jet type, comprising a generally elongated inner member defining a body of revolution relative to the longitudinal axis of said engine, a generally tubular outer cowl member formed with a progressively decreasing diameter to provide a restricted passage, said area of decreasing diameter forming an air intake port, having a parabolic inner surface, forwardly of said elongated member, said outer cowl member being symmetrically disposed with respect to said inner member, the parabolic surface of said outer cow-l member forwardly of said restricted passage reflecting radiant energy rays, initially impacting the parabolic surface of said outer cowl member after arriving at said parabolic surface along paths substantially parallel to the longitudinal axis of said engine, to a common focal point located within said inner member, said inner member being at least in part radiant-energy-permeable to pass therethrough the rays reflected thereto from the inner parabolic surface at said cowl.

References (Iited in the file of this patent .UNITED STATES PATENTS 2,540,594 Price Feb. 6, 1951 2,649,266 Darrieus Aug. 18, 1953 2,704,513 Dane Mar. 22, 1955 FOREIGN PATENTS 731,849 Germany Feb. 16, 1943 747,705 Great Britain Apr. 11, 1956 

